A variety of switchable coatings are known in the art. Oftentimes they are referred to as smart glass or switchable glass. Smart glass or switchable glass is glass or glazing whose light transmission properties are altered when voltage, light, heat and/or gas is applied. Generally, the glass changes from translucent or opaque to transparent, i.e., changing from scattering or blocking some, or all wavelengths of light to letting light pass through. Smart glass technologies include electrochromic, photochromic, thermochromic, gasochromic suspended particle, micro-blind and polymer dispersed liquid crystal devices, for example.
In electrochromic devices, the optical transmittance of the electrochromic material can be changed reversibly by the application of current or a voltage. In thermochromic devices, the optical transmittance of the thermochromic material can be changed depending on the temperature. In gasochromic devices, the optical transmittance of the gasochromic material can be changed by controlling the atmosphere.
In some smart glass, a switchable layer having chromic properties is used. The switchable layer controls light by absorbing light. For these types of chromic materials, they change their opacity between a translucent state to a transparent state. A drawback of such materials is that because they absorb light, the layer is heated and this heat is radiated back into the room or space fitted with the glass. To avoid such a drawback, it is desirable for the switchable layer to control the light by reflecting it instead of absorbing it.
Recent advances in electrochromic materials related to the use of transition-metal hydride electrochromics have led to the development of reflective hydrides which become reflective instead of absorbing and thus switch states between transparent and mirror-like. Using such hydrides, the switchable layer can be reversibly switched between a transparent state by hydrogenation and a mirror state (metal state) by dehydrogenation and/or somewhere therebetween.
The switchable layer characteristically comprises an alloy of one or more kinds of alkaline earth metal selected from calcium, strontium, barium, and magnesium, for example. Such alloys turn into a colorless, transparent state by storing hydrogen, and into a silver-colored mirror state by releasing hydrogen. Many magnesium binary alloys are known to be used as switchable layers such as MgTi, MgNi, MgCa and MgY, for example. Also, a ternary alloy MgZrNi has been used as a switchable layer with improved optical switching properties over binary alloys. The switchable layer may contain trace amounts of elements other than magnesium, calcium, strontium, and barium as an inevitable component.
As mentioned above, there are several methods of reversibly switching the switching layer. Two methods include gasochromic and electrochromic methods. In the gasochromic method, the switchable layer is exposed to a gas containing a concentration of hydrogen sufficient to induce hydrogenation (often around 0.5% hydrogen). For dehydrogenation, the switchable layer is exposed to a gas containing less than the required concentration of hydrogen (such as air). In the electrochromic method, hydrogen is driven into or out of the switchable layer by application of an electric field, causing the switchable layer to reversibly hydrogenate or dehydrogenate, depending on the polarity of the electric field.
FIG. 1 is a schematic cross-sectional view of a switchable device according to the prior art that can be incorporated into a gasochromic or electrochromic system. The switchable device 10 includes, in the following order, a substrate 12, a switchable layer 14 and a catalytic layer 16. The substrate 12 may be glass or a plastic, for example. The switchable layer 14 may be a magnesium binary alloy or the ternary alloy MgZrNi, for example. The catalytic layer 16 may be palladium, for example.
The switchable device 10 shown in FIG. 1 may be incorporated in an electrochromic switching device comprising a multilayer of solid thin films on a transparent glass or plastic substrate. An example is shown in FIG. 2. On the substrate 212 are deposited the following layers, moving away from the substrate 212: an optical switch layer 214, an optional barrier layer (not shown), a proton injector layer, also referred to as a catalytic layer 216, an optional barrier layer (not shown), a solid electrolyte layer 218, an ion storage layer 220, and a transparent conductor layer 222. While the layer stack is shown in one particular order, the arrangement of the layer stack may be reversed so that on the substrate 212 are deposited the following layers, moving away from the substrate: a transparent conductor layer 222, an ion storage layer 220, a solid electrolyte layer 218, an optional barrier layer, a catalytic layer 216, an optional barrier layer and an optical switch layer 214, for example. A voltage is applied between the transparent conductor 222 and switchable layer 214 The device requires a voltage to change to either state, for example, +5 V to go transparent, −5V to go reflective. At 0V it may remain at whatever state it was at, or it may drift towards a middle, “average” state.
More specifically, when a voltage is applied to the device, hydrogen protons in the ion storage layer 220 move to the optical switch layer 222 and the optical switch layer becomes hydrogenated as will be described hereinafter. When the voltage is reversed, the hydrogen protons move from the switch layer 222 back to the ion storage layer.
An example of such a typical multilayer stack according to FIG. 2 is: Mg4Ni/Pd/Al/Ta2O5/HxWO3/indium-tin-oxide (ITO) on a transparent substrate. If the optional barrier layers are included, the barrier layer may be comprised of Al2O3, TiO2, SnO2, SiAlN, or SiAlON and preferably each barrier layer is less than 50 Angstroms. When a positive voltage is applied to the ITO, protons in the HxWO3 ion storage layer move to the Mg4Ni optical switching layer and the Mg4Ni film becomes hydrogenated to form transparent MgH2 and Mg2NiH4 hydrides. When the voltage is reversed, the protons return to the ion storage layer and the Mg4Ni film is dehydrogenated.
The switchable device shown in FIG. 1 may also be incorporated in a gasochromic switching device. FIG. 3 is a schematic cross-section of a gasochromic switchable device according to the prior art. A gasochromic switchable device includes an optical switching layer 314 and a catalyst layer 316 disposed on a surface of a substrate 312 such as glass. A second substrate 312 is separated from the first substrate 312 by a gas space S which is sealed by sealers 330. Introduction of a gas including hydrogen into the gas space S by an atmosphere controller 340 causes the switchable layer 314 to switch from its metallic dehydrogenated state to its hydrogenated state. More particularly, when the gas including hydrogen is introduced into the space, the hydrogen reacts with the catalyst layer 316 and hydrogen is passed through to the switching layer 314 which becomes hydrogenated and thus transparent. When the hydrogen gas is removed and replaced with oxygen, the opposite happens and the switchable layer 314 becomes reflective.
It would be desirable to provide a switchable device which can be incorporated into an electrochromic or gasochromic device that has improved durability. Existing devices have suffered from short lifetimes due to oxidation of the switchable layer, loss of hydrogen from the system, diffusion of the catalyst into the switchable layer, or poisoning of the catalyst layer.
It would be desirable, for example, to provide a switchable device with improved optical switching properties such as speed of switching, possibilities for dimming and variable degrees of transparency.
It would be desirable to provide a switchable device that has improved optical transmittance in its transparent state,